1. Field of the invention
The present invention relates to an oscillator and, more particularly, to a ultra-high frequency oscillator (in the gigahertz range), or a microwave oscillator, having a GaAs-FET.
2. Description of the Prior Art
The microwave oscillator having a GaAs-FET is used, for example, in a receiver for satellite broadcasting. The oscillator employing a GaAs-FET can be divided into three types: a source grounded type; a gate grounded type; and a drain grounded type. The present invention is particularly related to the drain grounded type wherein one GaAs-FET is mounted on a dielectric substrate, such as on an aluminium oxide plate, together with a dielectric resonator that operates in the TE.sub.01.delta. mode. The electric connections on the substrate are effected by suitable striplines.
From another viewpoint, the microwave oscillators are classified into two types: the feedback type; and the negative resistance type, as disclosed in an article of Ishihara et al, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-28, No. 8, August 1980, pp. 817-824 (331-117D). The negative resistance type, which is also referred to as a band reflection type, takes the advantage in the points that its operation is stable and the structure is simple. The feedback type takes the advantage in the point that the oscillation frequency is stable even when the condition of the load or the power source changes.
The feedback type, however, has a drawback in the point that a strong magnetic coupling between the stripline and the dielectric resonator is required and, therefore, the locations of the striplines and the dielectric resonator must be adjusted at a high accuracy, resulting in a difficulty in the manufacturing process.
The oscillator of the present invention is particularly directed to the band reflection type (negative resistance type). An example of the prior art band reflection type oscillator is shown in FIG. 1. A GaAs-FET 10 has three terminals, gate G, source S and drain D. The drain D is connected to ground through a suitable capacitive element. The source S is connected to a matching circuit 12, and further to an output. The gate G is connected to a resonant circuit 14 defined by a stripline 2, resistor R1 and resonator DR made of a dielectric material. The resonator DR is provided in a magnetically coupled relationship with the stripline 2, thereby determining the oscillation frequency. FET 10 operates in the negative resistance region, wherein the current decreases as the voltage increases. In the circuit of FIG. 1, the feedback for the oscillation is carried out by a capacitance Cgs between gate and source, which is internally included in FET 10.
According to the prior art band reflection type oscillator described above, there is a disadvantage in that the oscillation frequency undesirably varies relative to a change in the source voltage or to a change in the condition of a connected load.